The present invention relates to a semiconductor device having a polycrystalline silicon layer to be used as a gate electrode or wiring layer and a method of manufacturing the same, and more particularly to a semiconductor device, in which the upper part of the polycrystalline silicon layer is formed of a compound with metals (hereinafter referred to as metal silicide) for reduction in electrical resistance, and a method of manufacturing the same.
A semiconductor device in which a polycrystalline silicon layer is employed as a gate electrode of an insulated gate field effect transistor (hereinafter referred to as IGFET) and a wiring layer connected thereto is widely used. However, such a device gives rise a problem that the polycrystalline silicon layer has a high electrical resistance. In a semiconductor memory device, for example, when a word line extending in one direction is made of a polycrystalline silicon layer, a portion on a gate insulating film is used as a gate electrode of IGFET, and a portion on a field insulating film is used as a wiring layer. Such a word line will not have low electrical resistance at a desired level. To solve this problem, there has been proposed a technique in which metal silicide is formed in the upper part of a polycrystalline silicon layer. According to this technique in the prior art, however, the metal silicide layer is possible to diffuse into the polycrystalline silicon layer and finally to reach the interface with a insulating film such as a gate insulating film due to various high-temperature heat treatment process carried out after forming of the metal silicide layer which are essential in manufacturing the semiconductor device. This phenomenon is resulted from the reason that the heat treatment at high temperature activates metal atoms and silicon atoms both forming the metal silicide layer, thus collapsing the two-layer structure consisting of a metal silicide layer and a polycrystalline silicon layer which was completely maintained initially after forming of the metal silicide layer.
If the metal silicide layer reaches the interface with the gate insulating film, IGFET would have a remarkable influence in its electrical characteristics. Namely, IGFET is operated by controlling the electric field applied to the channel region so as to determine the amount of current flowing therethrough. Whereas, if a metal silicide layer in the gate electrode is attached on the gate insulating film, the precise control for the electric field in the channel region cannot be realized. Therefore, electrical characteristics of IGFET are unavoidably fluctuated to a large extent. For example, more specifically, enhancement type IGFET will be subjected to large fluctuations in the threshold voltage for forming of the inversion layer.